In recent years, the generation of electric power using fossil fuels, particularly hydrocarbon fuel gases produced from coal gasification processes, has received careful scrutiny because of the potential adverse impact on the environment of atmospheric emissions containing sulfur compounds such as hydrogen sulfide. In the past, some processes for treating fossil fuels prior to combustion and/or gasification have achieved limited success in minimizing unwanted emissions. In addition, a number of post-combustion treatments of the gases (typically relying on complex solvent or other sorbent removal techniques to remove sulfur compounds) have proven technically feasible, although often not economical.
The use of high-sulfur content coal presents significant pollution concerns because the gasification process produces a raw fuel gas comprised of carbon monoxide, hydrogen, and lesser quantities of carbon dioxide (CO2), methane and gaseous sulfur compounds, principally hydrogen sulfide (H2S) and carbonyl sulfide (COS). Small amounts of ammonia, elemental nitrogen, hydrogen chloride and hydrogen cyanide may also be present. From an environmental standpoint, it is important to remove such components from any emissions, particularly the sulfur compounds.
Many industries, such as refining and petrochemical plants, often generate waste streams that contain unconverted hydrocarbons, free hydrogen, sulfide, carbon oxides and inert gases. Such waste streams typically occur at the end of refining operations and have been used as residual fuel for other processes within the same refinery complex. Higher processing efficiencies can be realized if waste streams containing unconverted hydrocarbons (including olefin compounds) and hydrogen are economically converted to higher value hydrocarbon products for use as fuel or chemical feedstock, provided that waste components such as hydrogen sulfide are first removed.
Thus, processes capable of efficiently removing and then reusing waste streams containing sulfur compounds have become of considerable interest in the petrochemical, power generation and oil and gas refining industries. Environmental concerns over waste streams have also resulted in an increased interest in removing hydrogen sulfide and/or recovering elemental sulfur without adversely affecting the efficiency of an overall commercial process.
One known process for treating gas streams containing hydrocarbons and sulfur waste components relies on the catalytic conversion of hydrocarbon constituents to hydrogen and carbon monoxide. Examples of such processes include catalytic steam reforming, catalytic partial oxidation, and catalytic autothermal reforming and non-catalytic partial oxidation. These known processes (often referred to as “synthesis gas” or “SynGas” processes) produce various ratios of hydrogen and carbon monoxide and can contain sulfur contaminants.
Heretofore, the treatment of synthesis gas containing hydrogen sulfide and carbon dioxide using direct oxidation of H2S has not been accomplished in a commercially efficient or economical manner. In the known modified Claus process, following the removal from the synthesis gas, a portion of the hydrogen sulfide is combusted with air or oxygen to form sulfur dioxide. The uncombusted hydrogen sulfide is then reacted with the sulfur dioxide to form elemental sulfur and water.
The following patents relate to the desulfurization of fuel gas. U.S. Pat. No. 4,552,572, issued to Bechtel International, describes a method and apparatus for removing acid gases from a first gas stream rich in carbon dioxide and for desulfurizing a second gas stream lean in carbon dioxide but containing sulfur compounds as impurities. The '572 process requires contacting the first gas with a solvent capable of extracting carbon dioxide and sulfur compounds from the first stream and thereafter separating the carbon dioxide and sulfur.
U.S. Pat. No. 3,824,766 describes an adiabatic process for purifying hydrocarbon gases containing CO2 and sulfur compounds as impurities. A CO2-laden solvent is used to extract sulfur from the impure gas stream. After removal of the sulfur compounds, the partly purified gas stream passes to an expansion turbine where work is produced by the resulting gas stream. The residual CO2 is eventually removed using solvent extraction.
U.S. Pat. No. 4,332,598 uses a CO2-laden physical solvent to extract the bulk of sulfur compounds from an impure gas. The partially purified gas is then contacted with a fully regenerated solvent to extract the CO2 along with the remaining sulfur compounds, mostly comprising COS. The CO2 is eventually removed from the system using downstream stripping and solvent extraction techniques.